In the manufacture of articles from rubbery, viscoelastic and thermoplastic working materials (e.g.: polymers, and certain thermosetting resins, vulcanizable materials and polymerzable monomers) all of which are sometimes called plastic stock, it is common practice to filter the working materials. This may be done, for example, in a reclaiming process, to prepare re-claimed materials for use in a product; it may also be done, for example, in an extrusion apparatus, to make certain that no foreign matter will be introduced into an extrusion die.
An extrusion process is a continuous process in which the quality and quantity of the extrudate are proportional to the stability and consistency of the melt. A change in operating conditions changes the quality or quantity or both of the end product, and some of the conditions which can change are affected by conventional filtering operations. The practice of filtering the working material in an extrusion process requires the positioning of a filter medium across the path of material flow resulting in a pressure drop. The size of the pressure drop is related to the difficulty with which working material passes through the filter medium. When the filter medium is clean the pressure drop across the medium is low as compared to the pressure drop which exists when the filter medium is clogged with collected contaminants. For example, a 1500 psi drop is typical for a clean filter, whereas a pressure drop of 4000 psi is typical for a clogged filter medium.
It is desirable to have a constant and preferably low pressure drop across the filter medium for reasons relating to the quality and quantity of the working material. When the pressure drop across the filter medium increases, the shear on the working material increases, which in turn increases the temperature of the working material. The temperature of the working material is a basic operating parameter which determines a particular quality of the material. In addition, an increase in pressure drop across the filter medium reduces the rate of production of the entire line as well as requiring additional extruder pump horsepower to overcome the rising back pressure. Of course, an increase in cost also results.
The need to minimize transients in operating conditions caused by the collection of contaminants on the filter medium has been recognized for more than 75 years. A rigid slide plate filter, not unlike filters in use today, is shown in U.S. Pat. No. 642,814 issued to Cowen in 1900. It has two filters on one slide plate, and when one of the filters becomes clogged the slide plate is moved to remove the first filter entirely and replace it with the second filter, bringing about an immediate dramatic change in operating pressure. Other forms of rigid slide plate filter like Cowen's are shown in Garrahan's U.S. Pat. No. 1,195,576; Voight U.S. Pat. No. 3,684,419; Paquette U.S. Pat. No. 3,797,655; Cooper et al U.S. Pat. No. 3,804,758; and Heston U.S. Pat. No. 3,983,038.
It is common practice to use slide plate filters in a process where the working material is highly pressurized, sometimes as high as 10,000 pounds per square inch or more. There is, therefore, a need to seal the slide filter apparatus against leakage of working material from the melt stream along surfaces of the slide plate. On the other hand, there is a need to move the filter medium across the melt stream at such a rate that a low, constant pressure drop is present, and if the sealing means used presents high friction between the slide plate and the filter housing, then the ability to move the filter medium across the path of working material flow (i.e.: the "melt stream") is made more difficult. Filters that are useable at such high operating pressures with sealing means that can be maintained continuously at the inlet port passageway and at the outlet port passageway while permitting the screen plate means carrying filter media to be moved continuously across the melt stream are described and claimed in copending applications of the present inventor and another, Ser. No. 712,962, filed Aug. 9, 1976; and Ser. No. 801,748, filed May 31, 1977.
In filters of the type described in the copending applications, a rigid self-supporting filter plate carries filter media across the melt stream from an inlet port passageway to an outlet port passageway, at least the outlet port passageway being larger in cross-section than the filter plate, and a seal of stiffened working material is formed in the gap between the filter plate and the walls of the passageway. The seal material, including on the up-stream face of the filter plate used filter media and contaminants filtered from the melt stream, emerges from the outlet port passageway with the filter plate as the latter is advanced across the melt stream. Typical practice in such filters is to use sections of filter plate means in succession, one following the other across the melt stream, and as each section emerges from the outlet port passageway to treat it as by heating to remove the hardened seal material and spent filter media and contaminants in order to prepare the filter plate section for re-use. This practice is time-consuming, and it adds cost to the filter process.